1. Field of the Invention
The present invention relates to a developing device used in a copy machine, a facsimile machine, and a printer. More particularly, the present invention relates to a developing device that uses a two-component developer consisting of toner and a carrier and to an image forming apparatus that uses the developing device.
2. Description of the Related Art
A developing device of this type usually causes a two-component developer (hereinafter, “developer”), which is conveyed in a supplying-conveying path extending along the direction of the rotation axis of a developer carrier, to be carried on the surface of the developer carrier. By moving the two-component developer on the surface of the developer carrier, the developing device supplies the two-component developer to a developing area opposed to a latent image carrier. In the developing area, a latent image formed on the latent image carrier is developed to a visible toner image. The developer, from which toner has been consumed to develop the toner image in the developing area, is collected from the surface of the developer carrier, mixed with resupplied toner and stirred, and then re-used for further developing.
Such a developing device includes a circulating-conveying path along which the developer that has reached the conveying-direction posterior end of the supplying-conveying path is conveyed to the conveying-direction anterior end of the supplying-conveying path.
Japanese Patent No. 3494963, Japanese Patent No. 4333057, and Japanese Patent Application Laid-open No. 2009-98286 describe a configuration in which a supplying-conveying path and a circulating-conveying path, which extends along the direction of the rotation axis of a developer carrier and conveys a developer in a direction opposite to that of the supplying-conveying path, are arranged vertically. This configuration reduces the size of the developing device in the horizontal direction compared to a configuration in which the circulating-conveying path and the supplying-conveying path are arranged horizontally. This arrangement is not limited to a circulating-conveying path and a supplying-conveying path. By arranging any two developer conveying paths vertically, the size of the developing device in the horizontal direction can be reduced.
Furthermore, Japanese Patent No. 3494963 describes a developing device that includes a circulating-conveying path that is arranged above a supply conveying path and also describes a developer passing unit that conveys upward a developer that has reached a conveying-direction posterior end of a supplying-conveying path and passes the developer to the circulating-conveying path. In the configuration in which two conveying paths are arranged vertically, if passing of the developer from the lower conveying path to the upper conveying path depends on the pressure exerted by the horizontal conveying force of a conveying unit that is arranged in the lower conveying, path, the developer deteriorates easily. By providing a developer passing unit as described in Japanese Patent No. 3494963, the developer can be prevented from deteriorating when the developer is passed from the lower conveying path to the upper conveying path.
In a developing device that uses a two-component developer, in order to obtain a stable toner image, it is necessary to maintain the toner density or the charge of the toner within a predetermined range in the developer. The toner density in the developer can be adjusted by re-supplying a volume of toner corresponding to the volume of toner consumed during developing. The toner is charged by the triboelectric effect, which occurs when the carrier and the toner are mixed. By mixing the carrier and the toner sufficiently, an amount of charge within a desired range can be obtained. Even if the toner density in the developer in the developing device is within a predetermined range, but if the toner density distribution is uneven, the toner image density may also become uneven. For this reason, in the developing device, in order to make the toner density distribution even and to cause the toner to be charged by a desired amount and thus to stabilize the toner image, it is necessary to sufficiently stir the two-component developer consisting of the toner and the carrier. However, providing further additionally a stirring unit may result in a complicated or large developing device.